Genetics of incipient speciation in Drosophila mojavensis: II. Host plants and mating status influence cuticular hydrocarbon QTL expression and G x E interactions

Host Plant Effects on Sexual Selection Dynamics in Phytophagous Insects

Natural selection is notoriously dynamic in nature, and so, too, is sexual selection. The interactions between phytophagous insects and their host plants have provided valuable insights into the many ways in which ecological factors can influence sexual selection. In this review, we highlight recent discoveries and provide guidance for future work in this area. Importantly, host plants can affect both the agents of sexual selection (e.g., mate choice and male-male competition) and the traits under selection (e.g., ornaments and weapons). Furthermore, in our rapidly changing world, insects now routinely encounter new potential host plants. The process of adaptation to a new host may be hindered or accelerated by sexual selection, and the unexplored evolutionary trajectories that emerge from these dynamics are relevant to pest management and insect conservation strategies. Examining the effects of host plants on sexual selection has the potential to advance our fundamental understanding of sexual conflict, host range evolution, and speciation, with relevance across taxa.

Aging-Related Variation of Cuticular Hydrocarbons in Wild Type and Variant Drosophila melanogaster

The cuticle of all insects is covered with hydrocarbons which have multiple functions. Cuticular hydrocarbons (CHCs) basically serve to protect insects against environmental harm and reduce dehydration. In many species, some CHCs also act as pheromones. CHCs have been intensively studied in Drosophila species and more especially in D. melanogaster. In this species, flies produce about 40 CHCs forming a complex sex- and species-specific bouquet. The quantitative and qualitative pattern of the CHC bouquet was characterized during the first days of adult life but remains unexplored in aging flies. Here, we characterized CHCs during the whole-or a large period of-adult life in males and females of several wild type and transgenic lines. Both types of lines included standard and variant CHC profiles. Some of the genotypes tested here showed very dramatic and unexpected aging-related variation based on their early days' profile. This study provides a concrete dataset to better understand the mechanisms underlying the establishment and maintenance of CHCs on the fly cuticle. It could be useful to determine physiological parameters, including age and response to climate variation, in insects collected in the wild.

Plastic Variation in the Phyletic Lineages of Cactophilic Drosophila meridionalis and Relation to Hosts as Potential for Diversification

The insect/plant interaction is known to be a trigger for diversification and even speciation. Experimental analyses on fitness traits and phenotypic variation using alternative host sites have been performed to understand the process of diversification relative to insect/plant interactions. For cactophilic species of Drosophila, the speciation process is considered an adaptive radiation in response to the exploration of species of the Cactaceae as breeding and feeding sites. In this work, we analyzed life history and morphological traits in individuals from two phyletic lineages (Evolutionarily Significant Units ESU) of the cactophilic species Drosophila meridionalis (Wasserman 1962) (Diptera: Drosophilidae) raised from media prepare. The characters analyzed corresponded to viability, developmental time, and four morphological measurements. The experiments were performed in a semi-natural medium prepared with fermenting tissues of the natural hosts, Cereus hildmaniannus and Opuntia monacantha. Viability, development time, and three morphological measurements were influenced by lineage, suggesting differentiation between the lineages. However, in O. monacantha, the mean viability was greater (~15%) and development time was longer (~336 h) than in C. hildmaniannus (~11% and ~301 h, respectively). Only the developmental time was significantly affected by the host cactus. In general, ESU group A had better values than ESU group BC for the evaluated traits. This finding suggested differentiation between the two lineages and different plastic responsiveness to the contrasting environments of the hosts, and that C. hildmaniannus may be a relatively stressful environment for the larvae, as for other Drosophila species.

Genetics and Distribution Modeling: The Demographic History of the Cactophilic Drosophila buzzatii Species Cluster in Open Areas of South America

Biodiversity is the result of historical and recurrent events acting on populations and species. The Drosophila buzzatii species cluster is distributed along a diagonal of open areas in South America. Combining genetic analyses with species distribution models we evaluated the influence of climatic changes in the demography history of this cluster. We performed a Bayesian Skyline analysis and reconstructed the ancestral areas based on mitochondrial cytochrome oxidase I (mtCOI) gene. We modeled the current distribution and projected it to past (mid-Holocene and Last Glacial Maximum) and future. Our results demonstrate that climate change plays a critical role in historical demography and in defining the current and future geographic ranges of these species. The inter-Andean dry valleys and the Chiquitano Seasonally Dry Tropical Forests (SDTF) in Bolivia are considered the ancestral area for the D. buzzatii cluster. From this area, the migration route was through a west-east corridor from central Andes, throughout Bolivia, Paraguay and Argentina toward eastern and northeastern Brazil, along the edges of rain forest. The responses of the species to the climatic changes differ from the dominant assumptions of expansion during dry/cold weather and contraction during wet/warm weather that characterized the glacial cycles. We suggest that the influence of ecology and the potential responsiveness of each taxon to the environmental dynamics should be considered as well. Predictions for the future suggest a decline of suitable areas for the cluster, threatening biodiversity of these habitats. This work showed the importance of an integrative analysis of genetics and geography information to improve the inferences about demographic history hypotheses for the cluster.

Evolutionary genomics of host plant adaptation: insights from Drosophila

Variation in gene expression in response to the use of alternate host plants can reveal genetic and physiological mechanisms explaining why insect-host relationships vary from host specialism to generalism. Interpreting transcriptome variation relies on well-annotated genomes, making drosophilids valuable model systems, particularly those species with tractable ecological associations. Patterns of whole genome expression and alternate gene splicing in response to growth on different hosts have revealed expression of gene networks of known detoxification genes as well as novel functionally enriched genes of diverse metabolic and structural functions. Integrating trancriptomic responses with fitness differences and levels of phenotypic plasticity in response to alternate hosts will help to reveal the general nature of genotype-phenotype relationships.

Assessing the Architecture of Drosophila mojavensis Locomotor Evolution with Bulk Segregant Analysis

Behavior is frequently predicted to be especially important for evolution in novel environments. If these predictions are accurate, there might be particular patterns of genetic architecture associated with recently diverged behaviors. Specifically, it has been predicted that behaviors linked to population divergence should be underpinned by a few genes of relatively large effect, compared to architectures of intrapopulation behavioral variation, which is considered to be highly polygenic. More mapping studies of behavioral variation between recently diverged populations are needed to continue assessing the generality of these predictions. Here, we used a bulk segregant mapping approach to dissect the genetic architecture of a locomotor trait that has evolved between two populations of the cactophilic fly Drosophila mojavensis We created an F8 mapping population of 1,500 individuals from advanced intercross lines and sequenced the 10% of individuals with the highest and lowest levels of locomotor activity. Using three alternative statistical approaches, we found strong evidence for two relatively large-effect QTL that is localized in a region homologous to a region of densely packed behavior loci in Drosophila melanogaster, suggesting that clustering of behavior genes may display relatively deep evolutionary conservation. Broadly, our data are most consistent with a polygenic architecture, though with several loci explaining a high proportion of variation in comparison to similar behavioral traits. We further note the presence of several antagonistic QTL linked to locomotion and discuss these results in light of theories regarding behavioral evolution and the effect size and direction of QTL for diverging traits in general.

Mate choice and gene expression signatures associated with nutritional adaptation in the medfly (Ceratitis capitata)

Evolutionary responses to nutrition are key to understanding host shifts and the resulting potential for reproductive isolation. Experimental evolution has previously been used to describe the responses of the medfly (Ceratitis capitata) to larval diets with different nutritional properties. Within 30 generations this led to divergence in larval development time, egg to adult survival and adaptation in adult body size. Here we used mRNA-seq to identify differences in gene expression patterns in these same populations, using males from the 60^th generation of nutritional selection. We validated differential expression by using qRT-PCR and found that genes linked to metabolism, oxidative phosphorylation and proteolysis were significantly over-represented among the differentially expressed genes. The results provide the first genome-wide survey of the putative mechanisms underpinning evolved responses to nutritional adaptation. In addition, we tested the hypothesis that nutritional adaptation can alter mating patterns. We found evidence for assortative mating by diet at generation 60, but not 90. Hence, the pattern was variable across generations and there was no evidence overall for any isolating mating divergence between the lines. Overall, the results provide insight into the mechanisms underpinning dietary adaptation and extend our knowledge of which traits represent core responses to nutritional selection.

Mating system manipulation and the evolution of sex-biased gene expression in Drosophila

Sex differences in dioecious animals are pervasive and result from gene expression differences. Elevated sexual selection has been predicted to increase the number and expression of male-biased genes, and experimentally imposing monogamy on Drosophila melanogaster has led to a relative feminisation of the transcriptome. Here, we test this hypothesis further by subjecting another polyandrous species, D. pseudoobscura, to 150 generations of experimental monogamy or elevated polyandry. We find that sex-biased genes do change in expression but, contrary to predictions, there is usually masculinisation of the transcriptome under monogamy, although this depends on tissue and sex. We also identify and describe gene expression changes following courtship experience. Courtship often influences gene expression, including patterns in sex-biased gene expression. Our results confirm that mating system manipulation disproportionately influences sex-biased gene expression but show that the direction of change is dynamic and unpredictable.

Chemical Heterogeneity in Inbred European Population of the Invasive Hornet Vespa velutina nigrithorax

Invasive social insect populations that have been introduced to a new environment through a limited number of introduction events generally exhibit reduced variability in their chemical signatures (cuticular hydrocarbons) compared to native populations of the same species. The reduced variability in these major recognition cues could be caused by a reduction of genetic diversity due to a genetic bottleneck. This hypothesis was tested in an inbred European population of the invasive hornet Vespa velutina nigrithorax. Our results show that, in spite of the limited amount of genetic diversity present in the European population, the chemical signatures of individuals were highly heterogeneous according to their caste, sex, and colony origin. In queens, some specific saturated and unsaturated hydrocarbons were identified. These results suggest that epigenetic and/or environmental factors could play a role in modifying cuticular hydrocarbon profiles in this introduced hornet population despite the observed reduction of genetic diversity.

Preadult life history variation determines adult transcriptome expression

Preadult determinants of adult fitness and behaviour have been documented in a variety of organisms with complex life cycles, but little is known about expression patterns of genes underlying these adult traits. We explored the effects of differences in egg-to-adult development time on adult transcriptome and cuticular hydrocarbon variation in order to understand the nature of the genetic correlation between preadult development time and premating isolation between populations of Drosophila mojavensis reared in different host cactus environments. Transcriptome variation was analysed separately in flies reared on each host and revealed that hundreds of genes in adults were differentially expressed (FDR P < 0.05) due to development time differences. For flies reared on pitaya agria cactus, longer preadult development times caused increased expression of genes in adults enriched for ribosome production, protein metabolism, chromatin remodelling and regulation of alternate splicing and transcription. Baja California flies reared on organ pipe cactus showed fewer differentially expressed genes in adults due to longer preadult development time, but these were enriched for ATP synthesis and the TCA cycle. Mainland flies reared on organ pipe cactus with shorter development times showed increased transcription of genes enriched for mitochondria and energy production, protein synthesis and glucose metabolism: adults with longer development times had increased expression of genes enriched for adult life span, cuticle proteins and ion binding, although most differentially expressed genes were unannotated. Differences due to population, sex, mating status and their interactions were also assessed. Adult cuticular hydrocarbon profiles also showed shifts due to egg-to-adult development time and were influenced by population and mating status. These results help to explain why preadult life history variation determines subsequent expression of the adult transcriptome along with traits involved with reproductive isolation and revealed previously undocumented connections between genetic and environmental influences over the entire life cycle in this desert insect.

Reproductive isolation among allopatric Drosophila montana populations

An outstanding goal in speciation research is to trace the mode and tempo of the evolution of barriers to gene flow. Such research benefits from studying incipient speciation, in which speciation between populations has not yet occurred, but where multiple potential mechanisms of reproductive isolation (RI: i.e., premating, postmating-prezygotic (PMPZ), and postzygotic barriers) may act. We used such a system to investigate these barriers among allopatric populations of Drosophila montana. In all heteropopulation crosses we found premating (sexual) isolation, which was either symmetric or asymmetric depending on the population pair compared. Postmating isolation was particularly strong in crosses involving males from one of the study populations, and while sperm were successfully transferred, stored, and motile, we experimentally demonstrated that the majority of eggs produced were unfertilized. Thus, we identified the nature of a PMPZ incompatibility. There was no evidence of intrinsic postzygotic effects. Measures of absolute and relative strengths of pre- and postmating barriers showed that populations differed in the mode and magnitude of RI barriers. Our results indicate that incipient RI among populations can be driven by different contributions of both premating and PMPZ barriers occurring between different population pairs and without the evolution of postzygotic barriers.

Wax, sex and the origin of species: Dual roles of insect cuticular hydrocarbons in adaptation and mating

Evolutionary changes in traits that affect both ecological divergence and mating signals could lead to reproductive isolation and the formation of new species. Insect cuticular hydrocarbons (CHCs) are potential examples of such dual traits. They form a waxy layer on the cuticle of the insect to maintain water balance and prevent desiccation, while also acting as signaling molecules in mate recognition and chemical communication. Because the synthesis of these hydrocarbons in insect oenocytes occurs through a common biochemical pathway, natural or sexual selection on one role may affect the other. In this review, we explore how ecological divergence in insect CHCs can lead to divergence in mating signals and reproductive isolation. We suggest that the evolution of insect CHCs may be ripe models for understanding ecological speciation.

Genome-wide tests for introgression between cactophilic Drosophila implicate a role of inversions during speciation

Models of speciation‐with‐gene‐flow have shown that the reduction in recombination between alternative chromosome arrangements can facilitate the fixation of locally adaptive genes in the face of gene flow and contribute to speciation. However, it has proven frustratingly difficult to show empirically that inversions have reduced gene flow and arose during or shortly after the onset of species divergence rather than represent ancestral polymorphisms. Here, we present an analysis of whole genome data from a pair of cactophilic fruit flies, Drosophila mojavensis and D. arizonae, which are reproductively isolated in the wild and differ by several large inversions on three chromosomes. We found an increase in divergence at rearranged compared to colinear chromosomes. Using the density of divergent sites in short sequence blocks we fit a series of explicit models of species divergence in which gene flow is restricted to an initial period after divergence and may differ between colinear and rearranged parts of the genome. These analyses show that D. mojavensis and D. arizonae have experienced postdivergence gene flow that ceased around 270 KY ago and was significantly reduced in chromosomes with fixed inversions. Moreover, we show that these inversions most likely originated around the time of species divergence which is compatible with theoretical models that posit a role of inversions in speciation with gene flow.

Identification of differentially expressed genes in female Drosophila antonietae and Drosophila meridionalis in response to host cactus odor

Background

Studies of insect-plant interactions have provided critical insights into the ecology and evolution of adaptive processes within and among species. Cactophilic Drosophila species have received much attention because larval development occurs in the necrotic tissues of cacti, and both larvae and adults feed on these tissues. Such Drosophila-cactus interactions include effects of the host plant on the physiology and behavior of the flies, especially so their nutritional status, mating condition and reproduction. The aim of this work was to compare the transcriptional responses of two species, Drosophila antonietae and Drosophila meridionalis, and identify genes potentially related to responses to odors released by their host cactus, Cereus hildmannianus. The two fly species are sympatric in most of their populations and use this same host cactus in nature.

Results

We obtained 47 unique sequences (USs) for D. antonietae in a suppression subtractive hybridization screen, 30 of these USs had matches with genes predicted for other Drosophila species. For D. meridionalis we obtained 81 USs, 46 of which were orthologous with genes from other Drosophila species. Functional information (Gene Ontology) revealed that these differentially expressed genes are related to metabolic processes, detoxification mechanisms, signaling, response to stimuli, and reproduction. The expression of 13 genes from D. meridionalis and 12 from D. antonietae were further analyzed by quantitative real time-PCR, showing that four genes were significantly overexpressed in D. antonietae and six in D. meridionalis.

Conclusions

Our results revealed the differential expression of genes related to responses to odor stimuli by a cactus, in two associated fly species. Although the majority of activated genes were similar between the two species, we also observed that certain metabolic pathways were specifically activated, especially those related to signaling pathways and detoxification mechanisms. The activation of these genes may reflect different metabolic pathways used by these flies in their interaction with this host cactus. Our findings provide insight into how the use of C. hildmannianus may have arisen independently in the two fly species, through genetic differentiation in metabolic pathways to effectively explore this cactus as a host.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0191-2) contains supplementary material, which is available to authorized users.

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. X. Age-specific dynamics of adult epicuticular hydrocarbon expression in response to different host plants

Analysis of sexual selection and sexual isolation in Drosophila mojavensis and its relatives has revealed a pervasive role of rearing substrates on adult courtship behavior when flies were reared on fermenting cactus in preadult stages. Here, we assessed expression of contact pheromones comprised of epicuticular hydrocarbons (CHCs) from eclosion to 28 days of age in adults from two populations reared on fermenting tissues of two host cacti over the entire life cycle. Flies were never exposed to laboratory food and showed significant reductions in average CHC amounts consistent with CHCs of wild-caught flies. Overall, total hydrocarbon amounts increased from eclosion to 14–18 days, well past age at sexual maturity, and then declined in older flies. Most flies did not survive past 4 weeks. Baja California and mainland populations showed significantly different age-specific CHC profiles where Baja adults showed far less age-specific changes in CHC expression. Adults from populations reared on the host cactus typically used in nature expressed more CHCs than on the alternate host. MANCOVA with age as the covariate for the first six CHC principal components showed extensive differences in CHC composition due to age, population, cactus, sex, and age × population, age × sex, and age × cactus interactions. Thus, understanding variation in CHC composition as adult D. mojavensis age requires information about population and host plant differences, with potential influences on patterns of mate choice, sexual selection, and sexual isolation, and ultimately how these pheromones are expressed in natural populations. Studies of drosophilid aging in the wild are badly needed.

Reproductive character displacement of epicuticular compounds and their contribution to mate choice in Drosophila subquinaria and Drosophila recens

Interactions between species can alter selection on sexual displays used in mate choice within species. Here we study the epicuticular pheromones of two Drosophila species that overlap partially in geographic range and are incompletely reproductively isolated. Drosophila subquinaria shows a pattern of reproductive character displacement against Drosophila recens, and partial behavioral isolation between conspecific sympatric versus allopatric populations, whereas D. recens shows no such variation in mate choice. First, using manipulative perfuming experiments, we show that females use pheromones as signals for mate discrimination both between species and among populations of D. subquinaria. Second, we show that patterns of variation in epicuticular compounds, both across populations and between species, are consistent with those previously shown for mating probabilities: pheromone compositions differ between populations of D. subquinaria that are allopatric versus sympatric with D. recens, but are similar across populations of D. recens regardless of overlap with D. subquinaria. We also identify differences in pheromone composition among allopatric regions of D. subquinaria. In sum, our results suggest that epicuticular compounds are key signals used by females during mate recognition, and that these traits have diverged among D. subquinaria populations in response to reinforcing selection generated by the presence of D. recens.

Barriers to gene exchange in hybridizing field crickets: the role of male courtship effort and cuticular hydrocarbons

Background

Pre-zygotic barriers often involve some form of sexual selection, usually interpreted as female choice, as females are typically the choosier sex. However, males typically show some mate preferences, which are increasingly reported. Here we document previously uncharacterized male courtship behavior (effort and song) and cuticular hydrocarbon (CHC) profiles in the hybridizing crickets Gryllus firmus and G. pennsylvanicus. These two species exhibit multiple barriers to gene exchange that act throughout their life history, including a behavioral barrier that results in increased time to mate in heterospecific pairs.

Results

We demonstrated that male mate choice (as courtship effort allocation) plays a more important role in the prezygotic behavioral barrier than previously recognized. In gryllids females ultimately decide whether or not to mate, yet we found males were selective by regulating courtship effort intensity toward the preferred (conspecific) females. Females were also selective by mating with more intensely courting males, which happened to be conspecifics. We report no differences in courtship song between the two species and suggest that the mechanism that allows males to act differentially towards conspecific and heterospecific females is the cuticular hydrocarbon (CHC) composition. CHC profiles differed between males and females of both species, and there were clear differences in CHC composition between female G. firmus and G. pennsylvanicus but not between the males of each species.

Conclusion

Although many barriers to gene exchange are known in this system, the mechanism behind the mate recognition leading to reduced heterospecific mating remains unknown. The CHC profiles might be the phenotypic cue that allow males to identify conspecifics and thus to adjust their courtship intensity accordingly, leading to differential mating between species.

Sex-specific triacylglycerides are widely conserved in Drosophila and mediate mating behavior

Pheromones play an important role in the behavior, ecology, and evolution of many organisms. The structure of many insect pheromones typically consists of a hydrocarbon backbone, occasionally modified with various functional oxygen groups. Here we show that sex-specific triacylclyerides (TAGs) are broadly conserved across the subgenus Drosophila in 11 species and represent a novel class of pheromones that has been largely overlooked. In desert-adapted drosophilids, 13 different TAGs are secreted exclusively by males from the ejaculatory bulb, transferred to females during mating, and function synergistically to inhibit courtship from other males. Sex-specific TAGs are comprised of at least one short branched tiglic acid and a long linear fatty acyl component, an unusual structural motif that has not been reported before in other natural products. The diversification of chemical cues used by desert-adapted Drosophila as pheromones may be related to their specialized diet of fermenting cacti.

DOI:http://dx.doi.org/10.7554/eLife.01751.001

For animals, the ultimate purpose of life is to have sex, as nothing is more important than passing down your genes to future generations. A wide range of strategies are therefore employed throughout nature to maximize the chances of sexual success, from ostentatious courtship rituals to the subtle subliminal signals sent out using chemicals called pheromones. Plants and animals release pheromones to influence the behavior of other plants and animals, often without the recipient being aware of it.

Hundreds of different insect pheromones have been discovered. Fruit flies release a number of different pheromones, all with similar chemical structures. Now, Chin et al. have discovered that male flies belonging to several species of fruit fly that live in the desert release chemicals called triacylglycerides (TAGs), which are commonly used for energy storage by many organisms as pheromones. During sex, the male fly rubs the TAGs onto the body of the female, which makes her less attractive to other male flies for several hours, thus increasing his chances of parenthood and passing his genes to future generations.

TAGs are also found in other insect species, but have been largely overlooked as pheromones. Moreover, the TAGs discovered by Chin et al. have an unusual structure, not previously seen in nature, which may result from the diet of fermenting cacti the desert-dwelling fruit flies enjoy.

DOI:http://dx.doi.org/10.7554/eLife.01751.002

Cuticular hydrocarbons of Drosophila montana: geographic variation, sexual dimorphism and potential roles as pheromones

Sexual selection within populations can play an important role in speciation when divergence in mating signals and their corresponding preferences occur along different coevolutionary trajectories in different populations. In insects, one potential target of sexual selection is the blend of cuticular hydrocarbons (CHCs), which often show intra- and interspecific variation, sexual dimorphism and may act as pheromones. In Drosophila montana, a cold-adapted, circumboreal member of the Drosophila virilis species group, flies from different populations have been found to show significant premating isolation as well as variation in male mating signal (song) and female preference. While the role of male courtship song in mate choice has been studied extensively, CHCs in this species have received little attention. In this study, we identified most of the CHCs found on the cuticle of D. montana and characterized population divergence and sexual dimorphism of CHC profiles among flies established from three natural populations - one European and two North American. We also studied their potential role as pheromones by analyzing CHCs of flies used in female-choice mating experiments. We report significant population×sex effects on CHC profiles, as well as significant relationships between some CHC principal components and particular mating behaviours, such as female attractiveness and male mating success, providing evidence that CHCs may play a role in mate choice in this species. The study also provides evidence for variation in the degree to which CHCs play a role in chemical communication among these populations, which may have an influence on the speciation process itself, and could be due to variation in interactions with other closely-related species that occur sympatrically with D. montana in some, but not other, parts of its distribution.

Endocrine control of cuticular hydrocarbon profiles during worker-to-soldier differentiation in the termite Reticulitermes flavipes

The social organization of termites, unlike that of other social insects, is characterized by a highly plastic caste system. With the exception of the alates, all other individuals in a colony remain at an immature stage of development. Workers in particular remain developmentally flexible; they can switch castes to become soldiers or neotenics. Juvenile hormone (JH) is known to play a key role in turning workers into soldiers. In this study, we analyzed differences in cuticular hydrocarbon (CHC) profiles among castes, paying particular attention to the transition of workers to soldiers, in the subterranean termite species Reticulitermes flavipes. CHCs have a fundamental function in social insects as they serve as cues in inter- and intraspecific recognition. We showed that (1) the CHC profiles of the different castes (workers, soldiers, nymphs and neotenics) are different and (2) when workers were experimentally exposed to a JH analog and thus induced to become soldiers, their CHC profiles were modified before and after the worker-presoldier molt and before and after the presoldier-soldier molt.

No boundaries: genomes, organisms, and ecological interactions responsible for divergence and reproductive isolation

Revealing the genetic basis of traits that cause reproductive isolation, particularly premating or sexual isolation, usually involves the same challenges as most attempts at genotype-phenotype mapping and so requires knowledge of how these traits are expressed in different individuals, populations, and environments, particularly under natural conditions. Genetic dissection of speciation phenotypes thus requires understanding of the internal and external contexts in which underlying genetic elements are expressed. Gene expression is a product of complex interacting factors internal and external to the organism including developmental programs, the genetic background including nuclear-cytotype interactions, epistatic relationships, interactions among individuals or social effects, stochasticity, and prevailing variation in ecological conditions. Understanding of genomic divergence associated with reproductive isolation will be facilitated by functional expression analysis of annotated genomes in organisms with well-studied evolutionary histories, phylogenetic affinities, and known patterns of ecological variation throughout their life cycles. I review progress and prospects for understanding the pervasive role of host plant use on genetic and phenotypic expression of reproductive isolating mechanisms in cactophilic Drosophila mojavensis and suggest how this system can be used as a model for revealing the genetic basis for species formation in organisms where speciation phenotypes are under the joint influences of genetic and environmental factors.

Insect speciation rules: unifying concepts in speciation research

The study of speciation is concerned with understanding the connection between causes of divergent evolution and the origin and maintenance of barriers to gene exchange between incipient species. Although the field has historically focused either on examples of recent divergence and its causes or on the genetic basis of reproductive isolation between already divergent species, current efforts seek to unify these two approaches. Here we integrate these perspectives through a discussion of recent progress in several insect speciation model systems. We focus on the evolution of speciation phenotypes in each system (i.e., those phenotypes causally involved in reducing gene flow between incipient species), drawing an explicit connection between cause and effect (process and pattern). We emphasize emerging insights into the genomic architecture of speciation as well as timely areas for future research.

Sex-specific responses to sexual familiarity, and the role of olfaction in Drosophila

Studies of mating preferences have largely neglected the potential effects of individuals encountering their previous mates (‘directly sexually familiar’), or new mates that share similarities to previous mates, e.g. from the same family and/or environment (‘phenotypically sexually familiar’). Here, we show that male and female Drosophila melanogaster respond to the direct and phenotypic sexual familiarity of potential mates in fundamentally different ways. We exposed a single focal male or female to two potential partners. In the first experiment, one potential partner was novel (not previously encountered) and one was directly familiar (their previous mate); in the second experiment, one potential partner was novel (unrelated, and from a different environment from the previous mate) and one was phenotypically familiar (from the same family and rearing environment as the previous mate). We found that males preferentially courted novel females over directly or phenotypically familiar females. By contrast, females displayed a weak preference for directly and phenotypically familiar males over novel males. Sex-specific responses to the familiarity of potential mates were significantly weaker or absent in Orco¹ mutants, which lack a co-receptor essential for olfaction, indicating a role for olfactory cues in mate choice over novelty. Collectively, our results show that direct and phenotypic sexual familiarity is detected through olfactory cues and play an important role in sex-specific sexual behaviour.

Interaction between temperature and male pheromone in sexual isolation in Drosophila melanogaster

In Drosophila, female hydrocarbons are known to be involved in premating isolation between different species and pheromonal races. The role of male-specific hydrocarbon polymorphism is not as well documented. The dominant cuticular hydrocarbon (CHC) in male D. melanogaster is usually 7-tricosene (7-T), with the exception of African populations, in which 7-pentacosene (7-P) is dominant. Here, we took advantage of a population from the Comoro Islands (Com), in which males fell on a continuum of low to high levels of 7-T, to perform temperature selection and selection on CHCs’ profiles. We conducted several experiments on the selected Com males to study the plasticity of their CHCs in response to temperature shift, their role in resistance to desiccation and in sexual selection. We then compared the results obtained for selected lines to those from three common laboratory strains with different and homogenous hydrocarbon profiles: CS, Cot and Tai. Temperature selection modified the CHC profiles of the Com males in few generations of selection. We showed that the 7-P/7-T ratio depends on temperature with generally more 7-P at higher temperatures and observed a relationship between chain length and resistance to desiccation in both temperature- and phenotypically selected Com lines. There was partial sexual isolation between the flies with clear-cut phenotypes within the phenotypically selected lines and the laboratory strains. These results indicate that the dominant male pheromones are under environmental selection and may have played a role in reproductive isolation.

Hydrocarbon divergence and reproductive isolation in Timema stick insects

Background

Individuals commonly prefer certain trait values over others when choosing their mates. If such preferences diverge between populations, they can generate behavioral reproductive isolation and thereby contribute to speciation. Reproductive isolation in insects often involves chemical communication, and cuticular hydrocarbons, in particular, serve as mate recognition signals in many species. We combined data on female cuticular hydrocarbons, interspecific mating propensity, and phylogenetics to evaluate the role of cuticular hydrocarbons in diversification of Timema walking-sticks.

Results

Hydrocarbon profiles differed substantially among the nine analyzed species, as well as between partially reproductively-isolated T. cristinae populations adapted to different host plants. In no-choice trials, mating was more likely between species with similar than divergent hydrocarbon profiles, even after correcting for genetic divergences. The macroevolution of hydrocarbon profiles, along a Timema species phylogeny, fits best with a punctuated model of phenotypic change concentrated around speciation events, consistent with change driven by selection during the evolution of reproductive isolation.

Conclusion

Altogether, our data indicate that cuticular hydrocarbon profiles vary among Timema species and populations, and that most evolutionary change in hydrocarbon profiles occurs in association with speciation events. Similarities in hydrocarbon profiles between species are correlated with interspecific mating propensities, suggesting a role for cuticular hydrocarbon profiles in mate choice and speciation in the genus Timema.

Transcriptome-wide expression variation associated with environmental plasticity and mating success in cactophilic Drosophila mojavensis

Ecological speciation occurs with the adaptation of populations to different environments and concurrent evolution of reproductive isolation. Phenotypic plasticity might influence both ecological adaptation and reproductive traits. We examined environment-specific gene expression and male mating success in cactophilic Drosophila mojavensis using transcriptome sequencing. This species exhibits cactus-dependent mating success across different species of host plants, with genotype-by-environment interactions for numerous traits. We cultured flies from egg to eclosion on two natural cactus hosts and surveyed gene expression in adult males that were either successful or unsuccessful in achieving copulation in courtship trials. We identified gene expression differences that included functions involved with metabolism, most likely related to chemical differences between host cactus species. Several epigenetic-related functions were identified that might play a role in modulating gene expression in adults due to host cactus effects on larvae, and mating success. Cactus-dependent mating success involved expression differences of genes implicated in translation, transcription, and nervous system development. This suggests a role of neurological function genes in the mating success of D. mojavensis males. Together, these results suggest that the influence of environmental variation on mating success via regulation of gene expression might be an important aspect of ecological speciation.

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. IX. Host plant and population specific epicuticular hydrocarbon expression influences mate choice and sexual selection

Sexual signals in cactophilic Drosophila mojavensis include cuticular hydrocarbons (CHCs), contact pheromones that mediate female discrimination of males during courtship. CHCs, along with male courtship songs, cause premating isolation between diverged populations, and are influenced by genotype × environment interactions caused by different host cacti. CHC profiles of mated and unmated adult flies from a Baja California and a mainland Mexico population of D. mojavensis reared on two host cacti were assayed to test the hypothesis that male CHCs mediate within-population female discrimination of males. In multiple choice courtship trials, mated and unmated males differed in CHC profiles, indicating that females prefer males with particular blends of CHCs. Mated and unmated females significantly differed in CHC profiles as well. Adults in the choice trials had CHC profiles that were significantly different from those in pair-mated adults from no-choice trials revealing an influence of sexual selection. Females preferred different male CHC blends in each population, but the influence of host cactus on CHC variation was significant only in the mainland population indicating population-specific plasticity in CHCs. Different groups of CHCs mediated female choice-based sexual selection in each population suggesting that geographical and ecological divergence has the potential to promote divergence in mate communication systems.

Dietary effects on cuticular hydrocarbons and sexual attractiveness in Drosophila

Dietary composition is known to have profound effects on many aspects of animal physiology, including lifespan, general health, and reproductive potential. We have previously shown that aging and insulin signaling significantly influence the composition and sexual attractiveness of Drosophila melanogaster female cuticular hydrocarbons (CHCs), some of which are known to be sex pheromones. Because diet is intimately linked to aging and to the activity of nutrient-sensing pathways, we asked how diet affects female CHCs and attractiveness. Here we report consistent and significant effects of diet composition on female CHC profiles across ages, with dietary yeast and sugar driving CHC changes in opposite directions. Surprisingly, however, we found no evidence that these changes affect female attractiveness. Multivariate comparisons among responses of CHC profiles to diet, aging, and insulin signaling suggest that diet may alter the levels of some CHCs in a way that results in profiles that are more attractive while simultaneously altering other CHCs in a way that makes them less attractive. For example, changes in short-chain CHCs induced by a high-yeast diet phenocopy changes caused by aging and by decreased insulin signaling, both of which result in less attractive females. On the other hand, changes in long-chain CHCs in response to the same diet result in levels that are comparable to those observed in attractive young females and females with increased insulin signaling. The effects of a high-sugar diet tend in the opposite direction, as levels of short-chain CHCs resemble those in attractive females with increased insulin signaling and changes in long-chain CHCs are similar to those caused by decreased insulin signaling. Together, these data suggest that diet-dependent changes in female CHCs may be sending conflicting messages to males.

A sexual ornament in chickens is affected by pleiotropic alleles at HAO1 and BMP2, selected during domestication

Domestication is one of the strongest forms of short-term, directional selection. Although selection is typically only exerted on one or a few target traits, domestication can lead to numerous changes in many seemingly unrelated phenotypes. It is unknown whether such correlated responses are due to pleiotropy or linkage between separate genetic architectures. Using three separate intercrosses between wild and domestic chickens, a locus affecting comb mass (a sexual ornament in the chicken) and several fitness traits (primarily medullary bone allocation and fecundity) was identified. This locus contains two tightly-linked genes, BMP2 and HAO1, which together produce the range of pleiotropic effects seen. This study demonstrates the importance of pleiotropy (or extremely close linkage) in domestication. The nature of this pleiotropy also provides insights into how this sexual ornament could be maintained in wild populations.

The genetic analysis of phenotypes and the identification of the causative underlying genes remain central to molecular and evolutionary biology. By utilizing the domestication process, it is possible to exploit the large differences between domesticated animals and their wild counterparts to study both this and the mechanism of domestication itself. Domestication has been central to the advent of modern civilization; and yet, despite domesticated animals displaying similar adaptations in morphology, physiology, and behaviour, the genetic basis of these changes are unknown. In addition, though sexual selection theory has been the subject of a vast amount of study, very little is known about which genes are underpinning such traits. We have generated multiple intercrosses and advanced intercrosses based on wild-derived and domestic chickens to fine-map genomic regions affecting a sexual ornament. These regions have been over-laid with putative selective sweeps identified in domestic chickens and found to be significantly associated with them. By using expression QTL analysis, we show that two genes in one region, HAO1 and BMP2, are controlling multiple aspects of the domestication phenotype, from a sexual ornament to multiple life history traits. This demonstrates the importance of pleiotropy (or extremely close linkage) in controlling these genetic changes.

Model-based comparisons of phylogeographic scenarios resolve the intraspecific divergence of cactophilic Drosophila mojavensis

The cactophilic fly Drosophila mojavensis exhibits considerable intraspecific genetic structure across allopatric geographic regions and shows associations with different host cactus species across its range. The divergence between these populations has been studied for more than 60years, yet their exact historical relationships have not been resolved. We analysed sequence data from 15 intronic X-linked loci across populations from Baja California, mainland Sonora-Arizona and Mojave Desert regions under an isolation-with-migration model to assess multiple scenarios of divergence. We also compared the results with a pre-existing sequence data set of eight autosomal loci. We derived a population tree with Baja California placed at its base and link their isolation to Pleistocene climatic oscillations. Our estimates suggest the Baja California population diverged from an ancestral Mojave Desert/mainland Sonora-Arizona group around 230,000-270,000years ago, while the split between the Mojave Desert and mainland Sonora-Arizona populations occurred one glacial cycle later, 117,000-135,000years ago. Although we found these three populations to be effectively allopatric, model ranking could not rule out the possibility of a low level of gene flow between two of them. Finally, the Mojave Desert population showed a small effective population size, consistent with a historical population bottleneck. We show that model-based inference from multiple loci can provide accurate information on the historical relationships of closely related groups allowing us to set into historical context a classic system of incipient ecological speciation.

Sexual selection on song and cuticular hydrocarbons in two distinct populations of Drosophila montana

Sexual selection has the potential to contribute to population divergence and speciation. Most studies of sexual selection in Drosophila have concentrated on a single signaling modality, usually either courtship song or cuticular hydrocarbons (CHCs), which can act as contact pheromones. We have examined the relationship between both signal types and reproductive success using F(1-3) offspring of wild-collected flies, raised in the lab. We used two populations of the Holarctic species Drosophila montana that represent different phylogeographic clades that have been separate for ca. 0.5 million years (MY), and differ to some extent in both traits. Here, we characterize the nature and identify the targets of sexual selection on song, CHCs, and both traits combined within the populations. Three measures of courtship outcome were used as fitness proxies. They were the probability of mating, mating latency, and the production of rejection song by females, and showed patterns of association with different traits that included both linear and quadratic selection. Courtship song predicted courtship outcome better than CHCs and the signal modalities acted in an additive rather than synergistic manner. Selection was generally consistent in direction and strength between the two populations and favored males that sang more vigorously. Sexual selection differed in the extent, strength, and nature on some of the traits between populations. However, the differences in the directionality of selection detected were not a good predictor of population differences. In addition, a character previously shown to be important for species recognition, interpulse interval, was found to be under sexual selection. Our results highlight the complexity of understanding the relationship between within-population sexual selection and population differences. Sexual selection alone cannot predict differences between populations.

Sexual isolation between North American and Bogota strains of Drosophila pseudoobscura

Sexual isolation, the reduced ability of organisms of different species to successfully mate, is one of the reproductive barriers that prevent gene flow between different taxa. Various species-specific signals during courtship are attributed to the sexual isolation between them. Drosophila pseudoobscura has been widely used to study the behavioral and genetic mechanisms underlying selection for sexual isolation, as a model system for speciation. D. pseudoobscura and its sibling species, D. persmilis, live together in many locations but are reproductively isolated from one another. North American geographic strains of D. pseudoobscura from the American West mate at random. Several decades ago, D. pseudoobscura was collected in the vicinity of Bogota, Colombia, and later named the subspecies D. pseudoobscura bogotana. Nearly 5,000 matings were observed in this study. We analyzed mating behavior and cuticular hydrocarbon profiles as well as courtship within and between North American and Bogota strains of D. pseudoobscura. Here we report for the first time that Bogota strains of D. pseudoobscura do show statistically significant sexual isolation from North American strains. In addition, there are quantitative variations in cuticular hydrocarbons as well as in courtship behavior between Bogota and North American strains, and females of both North American and Bogota strains show strong preference for North American strain males having high mating propensities, suggesting that the Bogota strains are at an early stage that could lead to a separate species.

What do we need to know about speciation?

Speciation has been a major focus of evolutionary biology research in recent years, with many important advances. However, some of the traditional organising principles of the subject area no longer provide a satisfactory framework, such as the classification of speciation mechanisms by geographical context into allopatric, parapatric and sympatry classes. Therefore, we have asked where speciation research should be directed in the coming years. Here, we present a distillation of questions about the mechanisms of speciation, the genetic basis of speciation and the relationship between speciation and diversity. Our list of topics is not exhaustive; rather we aim to promote discussion on research priorities and on the common themes that underlie disparate speciation processes.

Variation in the male pheromones and mating success of wild caught Drosophila melanogaster

Drosophila melanogaster males express two primary cuticular hydrocarbons (male-predominant hydrocarbons). These act as sex pheromones by influencing female receptivity to mating. The relative quantities of these hydrocarbons vary widely among natural populations and can contribute to variation in mating success. We tested four isofemale lines collected from a wild population to assess the effect of intrapopulation variation in male-predominant hydrocarbons on mating success. The receptivity of laboratory females to males of the four wild-caught lines varied significantly, but not consistently in the direction predicted by variation in male-predominant hydrocarbons. Receptivity of the wild-caught females to laboratory males also varied significantly, but females from lines with male-predominant hydrocarbon profiles closer to a more cosmopolitan one did not show a correspondingly strong mating bias toward a cosmopolitan male. Among wild-caught lines, the male-specific ejaculatory bulb lipid, cis-vaccenyl acetate, varied more than two-fold, but was not associated with variation in male mating success. We observed a strong inverse relationship between the receptivity of wild-caught females and the mating success of males from their own lines, when tested with laboratory flies of the opposite sex.

Variations on a theme: diversification of cuticular hydrocarbons in a clade of cactophilic Drosophila

Background

We characterized variation and chemical composition of epicuticular hydrocarbons (CHCs) in the seven species of the Drosophila buzzatii cluster with gas chromatography/mass spectrometry. Despite the critical role of CHCs in providing resistance to desiccation and involvement in communication, such as courtship behavior, mating, and aggregation, few studies have investigated how CHC profiles evolve within and between species in a phylogenetic context. We analyzed quantitative differences in CHC profiles in populations of the D. buzzatii species cluster in order to assess the concordance of CHC differentiation with species divergence.

Results

Thirty-six CHC components were scored in single fly extracts with carbon chain lengths ranging from C₂₉ to C₃₉, including methyl-branched alkanes, n-alkenes, and alkadienes. Multivariate analysis of variance revealed that CHC amounts were significantly different among all species and canonical discriminant function (CDF) analysis resolved all species into distinct, non-overlapping groups. Significant intraspecific variation was found in different populations of D. serido suggesting that this taxon is comprised of at least two species. We summarized CHC variation using CDF analysis and mapped the first five CHC canonical variates (CVs) onto an independently derived period (per) gene + chromosome inversion + mtDNA COI gene for each sex. We found that the COI sequences were not phylogenetically informative due to introgression between some species, so only per + inversion data were used. Positive phylogenetic signal was observed mainly for CV1 when parsimony methods and the test for serial independence (TFSI) were used. These results changed when no outgroup species were included in the analysis and phylogenetic signal was then observed for female CV3 and/or CV4 and male CV4 and CV5. Finally, removal of divergent populations of D. serido significantly increased the amount of phylogenetic signal as up to four out of five CVs then displayed positive phylogenetic signal.

Conclusions

CHCs were conserved among species while quantitative differences in CHC profiles between populations and species were statistically significant. Most CHCs were species-, population-, and sex-specific. Mapping CHCs onto an independently derived phylogeny revealed that a significant portion of CHC variation was explained by species' systematic affinities indicating phylogenetic conservatism in the evolution of these hydrocarbon arrays, presumptive waterproofing compounds and courtship signals as in many other drosophilid species.

Genetics of incipient speciation in Drosophila mojavensis. III. Life-history divergence in allopatry and reproductive isolation

We carried out a three-tiered genetic analysis of egg-to-adult development time and viability in ancestral and derived populations of cactophilic Drosophila mojavensis to test the hypothesis that evolution of these life-history characters has shaped premating reproductive isolation in this species. First, a common garden experiment with 11 populations from Baja California and mainland Mexico and Arizona reared on two host species revealed significant host plant X region and population interactions for viability and development time, evidence for host plant adaptation. Second, replicated line crosses with flies reared on both hosts revealed autosomal, X chromosome, cytoplasmic, and autosome X cactus influences on development time. Viability differences were influenced by host plants, autosomal dominance, and X chromosomal effects. Many of the F(1) , F(2) , and backcross generations showed evidence of heterosis for viability. Third, a QTL analysis of male courtship song and epicuticular hydrocarbon variation based on 1688 Baja × mainland F(2) males also revealed eight QTL influencing development time differences. Mainland alleles at six of these loci were associated with longer development times, consistent with population-level differences. Eight G × E interactions were also detected caused by longer development times of mainland alleles expressed on a mainland host with smaller differences among Baja genotypes reared on the Baja host plant. Four QTL influenced both development time and epicuticular hydrocarbon differences associated with courtship success, and there was a significant QTL-based correlation between development time and cuticular hydrocarbon variation. Thus, the regional shifts in life histories that evolved once D. mojavensis invaded mainland Mexico from Baja California by shifting host plants were genetically correlated with variation in cuticular hydrocarbon-based mate preferences.

Male-specific transfer and fine scale spatial differences of newly identified cuticular hydrocarbons and triacylglycerides in a Drosophila species pair

We analyzed epicuticular hydrocarbon variation in geographically isolated populations of D. mojavensis cultured on different rearing substrates and a sibling species, D. arizonae, with ultraviolet laser desorption/ionization mass spectrometry (UV-LDI MS). Different body parts, i.e. legs, proboscis, and abdomens, of both species showed qualitatively similar hydrocarbon profiles consisting mainly of long-chain monoenes, dienes, trienes, and tetraenes. However, D. arizonae had higher amounts of most hydrocarbons than D. mojavensis and females of both species exhibited greater hydrocarbon amounts than males. Hydrocarbon profiles of D. mojavensis populations were significantly influenced by sex and rearing substrates, and differed between body parts. Lab food–reared flies had lower amounts of most hydrocarbons than flies reared on fermenting cactus substrates. We discovered 48 male- and species-specific hydrocarbons ranging in size from C₂₂ to C₅₀ in the male anogenital region of both species, most not described before. These included several oxygen-containing hydrocarbons in addition to high intensity signals corresponding to putative triacylglycerides, amounts of which were influenced by larval rearing substrates. Some of these compounds were transferred to female cuticles in high amounts during copulation. This is the first study showing that triacylglycerides may be a separate class of courtship-related signaling molecules in drosophilids. This study also extends the kind and number of epicuticular hydrocarbons in these species and emphasizes the role of larval ecology in influencing amounts of these compounds, many of which mediate courtship success within and between species.

A roadmap for bridging basic and applied research in forensic entomology

The National Research Council issued a report in 2009 that heavily criticized the forensic sciences. The report made several recommendations that if addressed would allow the forensic sciences to develop a stronger scientific foundation. We suggest a roadmap for decomposition ecology and forensic entomology hinging on a framework built on basic research concepts in ecology, evolution, and genetics. Unifying both basic and applied research fields under a common umbrella of terminology and structure would facilitate communication in the field and the production of scientific results. It would also help to identify novel research areas leading to a better understanding of principal underpinnings governing ecosystem structure, function, and evolution while increasing the accuracy of and ability to interpret entomological evidence collected from crime scenes. By following the proposed roadmap, a bridge can be built between basic and applied decomposition ecology research, culminating in science that could withstand the rigors of emerging legal and cultural expectations.

The Genetic Basis of Sexually Selected Variation

Sexually selected traits contribute greatly to phenotypic diversity, yet we have historically understood little about their genetic basis and how that basis may affect their evolution. Recent work in developmental and quantitative genetics has provided both mechanistic and statistical descriptions of genotype-phenotype maps for sexually selected traits. These studies expose generally complex genetic architectures; genotype-phenotype maps are polygenic with allelic effects that are pleiotropic and highly context-dependent. At the same time, developments in quantitative genetics have provided new insights into the microevolutionary potential of standing variation and indicate genetic constraints on the contemporary evolution of male sexually selected characters, mate preferences, and also male mating success itself. Understanding the extent to which these constraints are a function of genetic architecture will require a tighter integration of developmental, molecular, and quantitative genetic approaches in a variety of model systems. Emerging genomic technologies offer an unprecedented opportunity to deepen our understanding of sexual selection as an evolutionary process.

Male genital and wing morphology in the cactophilic sibling species Drosophila gouveai and Drosophila antonietae and their hybrids reared in different host plants

Cactophilic Drosophila flies are excellent models to study adaptation to a relatively narrow spectrum of potential host plants and host-driven evolutionary diversification. Previous studies suggested a complex genetic architecture of wing and male genital morphology in phylogenetically basal species of the D. buzzatii cluster. In this work, we investigate the effect of experimental hybridization and host plant shifts on male genital and wing morphology in D. gouveai Tidon-Sklorz and Sene and D. antonietae Tidon-Sklorz and Sene, a pair of more recently derived species. We explicitly tested the hypotheses that wing and male genital morphology in interspecific hybrids depend on the host plant in which flies were grown. Our study shows that cactus hosts exert a strong effect on genital and wing morphology and that hybrids can be clearly differentiated on the basis of wing and genital morphology from both parental species. However, the extent of morphological differentiation between hybrids and pure species as well as plasticity patterns varied across organs, suggesting a complex genetic architecture for the studied traits.